Using join tables in ruby on rails

Question

Lets say I have two databases: one for students and one for classes. I would like to be able to 'add' classes to a specific student and also be able to add students to a specific class. I assume I need to use a join table here but I am a little lost on how to use them. I would ultimately like to be able to do something like:

  @class.students.find(@student_id) 

and this would tell me if the student is in the class or not. I know the relationship between classes and students is 'has_many' and vice versa. Does doing 't.references :students' in the migrate files accomplish that? I tried adding that line to my migrate file and then tried finding something using the statement above and it gave me an error. I am new to RoR so I am not even sure what the best way to go about achieving this is. Any help is appreciated!

Answer 1

Everything true what @Jordan said, here the concrete steps to take:

1. Create a migration: rails g model CourseStudent creates a join model for the n:m relation, and the migration to the corresponding table.

2. Edit the migration file CreateCourseStudent so it contains the following:

   class CreateCourseStudent < ActiveRecord::Migration   def change     create_table :course_students do |t|        # Your code comes here       t.integer :student_id       t.integer :course_id        # Here comes the generated code        t.timestamps     end   end end 

3. Run the migration: rake db:migrate. As a result, the join table should now exist in your database.

4. Add to the models the following code

   class Course < ActiveRecord::Base   has_many :course_students   has_many :students, :through => :course_students end  class Student < ActiveRecord::Base   has_many :course_students   has_many :courses, :through => :course_students end  class CourseStudent < ActiveRecord::Base   belongs_to :student   belongs_to :course end 

You are now able to use the methods generated by the methods belongs_to and has_many:

• @course.students
• @student.courses

Try to find all the relevant facts and snippets in the Rails Guides, there you should find all information you need to get on track. Good luck!

Answer 2

This is an old question, but just in case anyone stumbles upon this like I did, you can now have the relationships has_and_belongs_to_many. So yes, you would create a join table:

create_join_table :students, :courses do |t|   t.integer :student_id   t.integer :course_id end 

And then in the models, you would say that a student has_and_belongs_to_many :courses And a course has_and_belongs_to_many :students. There is no need to make a third class called CourseStudent. This link has all this information